This proposal takes an innovative approach to address a critical question in innate immunity and cell biology: Where within infected cells are viruses detected? We have recently discovered that in addition to mitochondria, peroxisomes are signaling platforms for antiviral innate immune signaling. Peroxisome-mediated signaling occurs through the actions MAVS, an adaptor protein that receives signals from RIG-I, an RNA helicase that surveys the cytosol for viruses containing RNA genomes. MAVS signaling from peroxisomes induces an unusual interferon-independent signaling pathway that activates the rapid expression of antiviral factors. This signaling pathway is activated by diverse viruses such as influenza virus, vesicular stomatitis virus and mammalian reovirus, and is capable of restricting viral replication. Based on this discovery, we now seek to (1) determine how signaling from peroxisomes leads to the initiation of antiviral immunity, (2) determine if peroxisomal signaling is critical for the control of viruses that disrupt type I interferon expression, and how viral restriction is accomplished, and (3) characterize a novel negative regulator of RIG-I signaling that functions from peroxisomes and mitochondria. Our proposed studies have the potential to profoundly change our view of how antiviral immunity is organized within mammalian cells. This work may facilitate the design of novel therapeutics to manipulate the subcellular positioning of innate immune signaling molecules, helping to either trigger or interfere with an immune reaction.)